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This article in SSSAJ

  1. Vol. 46 No. 6, p. 1137-1143
    Received: June 22, 1982
    Accepted: July 19, 1982

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Electron Spin Resonance Investigation of Mn2+ Complexation in Natural and Synthetic Organics1

  1. M. B. McBride2



Mechanisms of Mn2+ bonding in soil organics and pure organic compounds were compared by using electron spin resonance (ESR) spectroscopy. The observation that Mn2+ in lower-than-cubic symmetry produces a very broad signal was used to quantitatively determine the degree of metal complexation by organics. As the pH was raised, multiligand molecules such as carboxymethylcellulose, citrate, and succinate formed complexes with Mn2+ as shown by a decrease of ESR signal intensity. However, monocarboxylic acids such as benzoate and salicylate do not readily form complexes with Mn2+. A fulvic acid extracted from acid soil possessed a very limited number of strong complexation sites which became available to Mn2+ only at pH values > 6.0. Polycarboxylic acids, including natural organics (peat, humic acid) and synthetic polymers (polymethacrylate, polygalacturonic acid) bond Mn2+ electrostatically as the hydrated ion at low pH, but complex increasing amounts of Mn2+ as the pH is raised. A comparison of the ESR characteristics of Mn2+ bound to soluble carboxylic acids, polymeric carboxylic acids, and strong acid compounds such as sulfonate resin is used to characterize the bonding behavior of Mn2+ in soil organics. The cooperative effort of several surface functional groups appears to be necessary to cause Mn2+ to form an inner-sphere complex with organic solids. Higher temperatures generally favor the complexation of Mn2+.

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